Conventionally, Japanese patent application Laid-Open No. 9-53503 (1997) discloses a control apparatus for an internal combustion engine which calculates a quantity of air aspirated into a cylinder thereof based upon in-cylinder pressures detected at two points during a compression stroke. The control apparatus for the internal combustion engine obtains a deviation between the in-cylinder pressures detected at the two points prior to ignition timing during the compression stroke, and reads out the quantity of the air in accordance with the obtained deviation from a map (table) in advance prepared. And the control apparatus injects into the cylinder fuel a quantity of which corresponds to the quantity of the air obtained as described above.
It is, however, not easy to produce a map defining with high accuracy a relation between the intake air quantity and the deviation in the in-cylinder pressures detected at the two points prior to the ignition timing during the compression stroke. Therefore, it is difficult to accurately obtain an intake air quantity in the conventional internal combustion engine.